Ketoconazole (KLZ) is a new, oral antifungal imidazole. Most of the published data concerning its pharmacokinetics have been obtained using a nonspecific and imprecise microbiological assay. We have developed a new HPLC assay for quantitation of KCZ in biological fluids to study the disposition of KCZ in patients and normal volunteers. We have compared the results of the bioassay and our HPLC procedure, and find our procedure sensitive, precise, and specific. Studies in progress include pharmacokinetics of KCZ at 3 different oral doses (200, 400, 600 mg) in normal volunteers and monitoring of serum KCZ levels in patatients in an attempt to correlate serum levels with therapeutic response. Serum level monitoring of KCZ is also performed in patients with abnormal hepatic metabolism, unsual disposition profiles, and/or those taking other drugs which may affect KCZ serum levels.